1. Field of the Invention
The present invention relates to a transmission circuit used for communication devices such as mobile phones and wireless LAN devices, and particularly to a transmission circuit, which operates with high efficiency and low distortion, and a communication device using the transmission circuit.
2. Description of the Background Art
Communication devices such as mobile phones and wireless
LAN devices are required to secure linearity of a transmission signal over a wide output level range and also to operate with low power consumption. For such a communication device, a transmission circuit which operates with high efficiency and low distortion is used. Hereinafter, conventional transmission circuits will be described.
One of the conventional transmission circuits is, for example, a transmission circuit which uses a modulation method such as a quadrature modulation method to generate a transmission signal (hereinafter, referred to as a quadrature modulation circuit). Since the quadrature modulation circuit is well known, the description thereof will be omitted. A conventional transmission circuit capable of outputting, more efficiently than the quadrature modulation circuit, a transmission signal having high linearity is, for example, a transmission circuit disclosed in FIG. 9 of Japanese Laid-Open Patent Publication No. 2004-266351 (hereinafter, referred to as Patent Document 1). FIG. 7 is a block diagram showing an exemplary configuration of a conventional transmission circuit 90 disclosed in Patent Document 1. In FIG. 7, the conventional transmission circuit 90 comprises an amplitude and phase extraction section 91, phase modulation section 92, amplifier section 93, output terminal 94 and an amplitude control section 95.
The amplitude and phase extraction section 91 extracts amplitude data and phase data from input data. The amplitude data is inputted to the amplitude control section 95. The amplitude control section 95 supplies a voltage to the amplifier section 93 in accordance with the amplitude data. The phase data is inputted to the phase modulation section 92. The phase modulation section 92 performs phase modulation on the inputted phase data to output a resultant signal as a phase-modulated signal. The phase-modulated signal is inputted to the amplifier section 93. The amplifier section 93 amplifies the phase-modulated signal in accordance with the voltage supplied from the amplitude control section 95. The signal amplified by the amplifier section 93 is outputted from the output terminal 94 as a transmission signal. The transmission circuit 90 which operates in the above manner is called a polar modulation circuit.
The conventional transmission circuit 90 has a problem that in the case where an output level of the transmission signal is varied over a wide range, the linearity of the transmission signal is not always secured. For example, in the case where the conventional transmission circuit 90 is applied to the UMTS specifications, the transmission circuit 90 is required to vary the output level over a range of 75 dB which is wider than the case where the transmission circuit is applied to GSM/EDGE specifications. In such a case, the amplitude control section 95 of the transmission circuit 90 is required to greatly vary the voltage to be supplied to the amplifier section 93. However, since a DC bias voltage, which causes internal elements of the amplitude control section 95 to operate, cannot be freely set in the transmission circuit 90, it is difficult to secure the linearity of the amplitude control section 95 when the DC bias voltage is near VCC or GND potential. For this reason, the conventional transmission circuit 90 cannot always secure the linearity of the transmission signal in the case where the output level of the transmission signal is varied over a wide range.